1. Field of the Invention
The present invention relates to the bending of glass sheets, and more particularly, to a method and apparatus for transferring a heat-softened glass sheet from a conveyor means onto an outline bending mould, i.e. a ring mould, raising the mould, and bending the sheet. The invention finds application in, but is not limited to, a shaping process known as press bending, in which a heat-softened glass sheet is pressed between opposed complementary shaping surfaces. This process is used principally for the bending of glass sheets to manufacture panes for automotive use.
2. Description of the Related Art
Strict requirements are placed upon the optical quality of automotive glass panes, principally from the safety aspect (to ensure an undistorted view through a vehicle window), but also from an aesthetic aspect, since optical faults may have an adverse effect on the external appearance of a vehicle. It goes without saying that automotive glass panes must be free from surface imperfections such as marks, which render the glass objectionable from both safety and aesthetic aspects.
The standard of optical quality required has increased over the years, and demands placed upon glass shaping processes have increased still more owing to the increasingly difficult shapes to which a glass sheet must be bent for modern vehicles while achieving increased optical quality. It is in fact more difficult to achieve good optical quality on a deeply bent shape, and also on a complex shape, for example, than on a shallow, or simple shape. A simple shape is one in which the glass sheet is bent around one or more axes in a single direction, whereas in a complex shape the sheet is bent about two sets of axes at right angles to each other.
Optical or physical distortion of a heat-softened glass sheet may be introduced at a variety of points in a bending process; for instance when the sheet is transferred from a conveyor means onto a ring mould, and then bent. A ring mould is a bending mould comprising a shaping rim which contacts only the periphery of the glass sheet.
Various different conveyor means have been used in glass bending processes, for instance, glass sheets may be conveyed on a roller hearth, i.e. sets of rollers, which may be straight or curved, continuous, segmented, or comprise several short discrete rollers. Discrete rollers may be so short as to constitute discs. Alternatively, sheets may be conveyed on a gas hearth, that is, on a cushion of gas, which may for example be hot air, or the gaseous products of burning a combustible gas. The gas forming such a gas cushion emanates from apertures in a straight or curved bed.
Transfer of the heat-softened sheet from the conveyor means onto a ring mould is particularly critical in the roller hearth process. When the sheet is considerably hotter than the rollers, the nearest pause of the glass on the rollers will cause optical distortion in the form of striations, resulting from the localised temperature change in the parts of the sheet in contact with the rollers. If the sheet is stationary for a longer period of time, or is especially hot, it will begin to slump around the rollers, producing a corrugated physical distortion of the sheet known as "roller wave". The first of these faults is especially likely to occur if transfer to the ring mould takes place `out of the heat`, i.e. outside the furnace in which the glass sheet was heated up to bending temperature.
From the above it will be clear that it is desirable to avoid any periods of time for which the heat-softened glass sheet is stationary on the conveyor rollers, i.e. it is desirable that the sheet be transferred to the ring mould as soon as the sheet is in the correct position relative to the ring mould.
It is also desirable to separate the operations of transfer of the sheet, and bending the sheet. This provides valuable flexibility in the relative timing of the operations.
It is known from a variety of documents including U.S. Pat. No. 5,286,271 to transfer a heat-softened glass sheet from rollers onto a ring mould by arranging the ring so that it can rise through the rollers from an initial position below the rollers to a second position above them, thereby lifting the sheet from the rollers. As an alternative, U.S. Pat. No. 5,286,271 discloses the possibility of moving the rollers downwards so that the sheet is placed upon the ring.
EP 364,415 discloses a process and apparatus for bending glass sheets in which conveyor rollers are provided between male and female bending moulds. The conveyor rollers are downwardly mobile, one roller descending in an opposite direction with respect to the adjacent roller. That is, the rollers incline in opposite directions, so that they all point downwards but each roller crosses adjacent rollers. The result is that they form a cradle to receive the sheet after it has been bent.
GB 2,162,170 discloses a press bending apparatus for glass sheets in which a sheet is bent and tempered in a single station by providing upper and lower moulds containing apertures through which cooling air may be blown. During tempering, the bent sheet is oscillated on an auxiliary ring to temper it uniformly. Before bending, the sheet is placed on the auxiliary ring by lowering conveyor rolls. The upper mould moves downwards to press the sheet into the lower mould, then both upper and lower moulds are retracted for tempering. This is to provide space for the auxiliary ring to oscillate, and cooling air to escape.
A disadvantage of bending and tempering in a single station is that for bending, the shaping surfaces need to be relatively hot, whereas tempering inevitably results in cooling of the shaping surfaces during quenching. Temperature cycling in the station, thermal inefficiency and long processing times per sheet ("cycle time") are the consequences.
Shaping the sheet by downward movement of the upper mould so as to press it against the lower mould (which is below the initial level of the conveyor rolls) necessitates a considerable downward travel by the upper mould. The upper mould, being normally a full-face mould of metal or refractory, tends to be heavy, and undergoes large accelerations and decelerations in the course of its reciprocating movement. A mechanism capable of actuating the upper mould is likely to be bulky, complex and expensive. If the actuating mechanism is of an insufficient specification, operation of the upper mould may be slow, contributing to the cycle time, and/or inaccurate in terms of timing or positioning relative to the lower mould.